The effects of pentamidine isethionate on the developing embryo and fetus have not been previously published. Pregnant Sprague-Dawley rats were given various doses of pentamidine during the period of embryogenesis. Animals were killed on days 18 to 20 of pregnancy and their fetuses were removed by hysterectomy. Autopsies were performed on all fetuses. There were significant differences among groups with regard to maternal weight gain and pregnancy resorption. More pregnancy resorptions were noted in the group that received normal human doses (4 mg/kg/day) of pentamidine than in the control group (p less than 0.05). One structural anomaly consisting of unilateral renal agenesis was noted in the 711 fetuses examined. Skeletal survey of fetal rats was unremarkable. Pentamidine was without teratogenic effects in rats when administered in doses similar to those recommended for adult humans; however, it appears to have an embryocidal effect when given in those same doses during embryogenesis.